In electrical power systems, higher voltage and current levels (e.g. 600V to 745 kV, or higher, and currents greater than about 5 amps) cannot be physically measured directly by use of a voltage or current meter for safety and practical reasons. As a result, intermediate devices called instrument transformers are used to step the voltage and current down to safe and useable levels. These instrument transformers are subdivided into current transformers (CTs) and potential (voltage) transformers (PTs). The CTs and PTs produce signals proportional to the higher level values so that, by using simple multiplying factors, the actual higher level values can be calculated for uses such as metering and protection.
As is known in the art, so-called FT switches (such as those manufactured and sold by ABB and GE) are used in the industry for working with CT and PT circuits.
FT switches are typically mounted on the front of switchboard panels at a substation. The FT switch usually has a base and cover that may be made of moulded polycarbonate or other such polymer. Insulating barriers are moulded into the base to insulate the switch units (poles) from one another.
FT switches typically have anywhere from one to ten poles (i.e. switch units). Conventionally, each pole (switch unit) is identified with a letter or other such identifier. Two different types of poles exist: (i) potential poles and (ii) current poles.
Potential poles have a single, non-shorting knife blade. Current poles are usually designed as pairs of knife blades, a non-shorting knife blade (“metering pole”) and a shorting knife blade (“shorting pole”). When the handle of the shorting blade is moved to the open position, the shorting blade disengages from the jaw. However, before the shorting blade disengages from the jaw, a bottom cam on the shorting blade makes contact with a shorting spring that shorts out the circuit.
Each pole (switch unit) of the FT switch typically includes its own finger-operated switch handle for independently operating each switch unit. Knife blade switches may also be ganged together (e.g. with a horizontal interlocking bar or other such mechanism) so that multiple poles can be opened or closed in unison.
A test plug is inserted into the FT switch once all of its poles are opened. These test plugs have anywhere from one to ten poles and allow injection “upwards” toward the jaw terminal of the switches, thereby permitting field personnel to inject towards meters and relays for testing purposes.
A separate source test plug isolates the external connections from the relay, meter or other equipment being tested. Such a test plug fits into the stationary contact jaws of an FT switch. The L-shaped test blade ensures accurate alignment between the test plug and the stationary contact jaws.
This separate source test plug connects the relay inputs and outputs to a set of binding posts on the top of the test plug and completely isolates the relay circuit from the external connections by means of an insulated barrier along the bottom of the plug. The external test circuits can then be connected to these binding posts, which are usually staggered for better accessibility. Before inserting the separate source test plug, all switch blades must be placed in the fully open position.
One enduring shortcoming of this prior-art technology is that the test plug only permits injection toward the jaw terminal of the switches, but not (in the opposite direction) toward the transformers.
In view of this shortcoming, an improvement on this prior-art test plug would thus be highly desirable.